Goldfish have their own unique personalities. Some goldfish are outgoing while others are shy. However, most goldfish are laid back and enjoy being hand fed and playing in their tank. If your goldfish begins to display aggressive behaviors, such as biting another goldfish in the tank, you must find out why. A goldfish may become aggressive because he doesn't have enough space in the tank, he's mating or he's sick.
Avoid Aquarium Overcrowding
Goldfish must have a minimum of 10 gallons for one fish. Therefore, two goldfish should be in a minimum of a 20-gallon tank. Overcrowding an aquarium may result in poor water quality, especially because goldfish produce large amounts of waste each day. If your fish do not have enough space, they may become stressed so try purchasing a larger tank with sufficient gallon capacity for your goldfish.
Ensure Good Water Quality
Goldfish require clean water to thrive. If the water is poor quality, the goldfish may become aggressive as a response to the unhealthy living conditions. Partial water changes should be done once a week or every two weeks, depending on the tank size and number of fish. Remove approximately 30 percent, or 3 gallons, of water for every 10 gallons in the tank. Replace with fresh water that has been treated with a water conditioner. Partial water changes will encourage the growth of good bacteria and ensure cleaner water.
There is only one [species](http://www.bristol-aquarists.org.uk/goldfish/goldfish.htm) of goldfish. However, there are many breeds -- such as fancy and bubble eye -- within the goldfish species.
Feed the Right Amount
Goldfish do not have stomachs. When they eat, the food does not remain in their bodies long and they produce more waste. Underfeeding your goldfish may result in aggressive behavior such as biting tank mates in an attempt to get more food. Overfeeding may result in your goldfish becoming sick. Feed your goldfish either two or three times a day and only feed as much as they will eat in two minutes. If you're feeding pellets or vegetables, for example, remove any food that your goldfish have not eaten after two minutes.
Consult an Aquatic Veterinarian
Your goldfish's fins may look as though they've been bitten. However, the common fish disease ich, or white spot disease, may be the culprit. A disease caused by parasitic protozoan, ich presents itself as small white spots on a fish's gills or skin. The rapidly reproducing parasites spread from fish to fish very quickly. Consult an aquatic veterinarian immediately if you suspect your fish has ich. Left untreated goldfish will die. An aquatic veterinarian will prescribe the correct treatment to curb the disease.
A Mating Ritual
Your goldfish may not be biting each other at all. If you notice your male goldfish chasing the female around the tank, he is ready to mate. If you don't want goldfish babies, remove the male and put him alone in a tank during breeding season in the spring.
To attempt to curb aggressive behavior, you can:
- Put an acrylic tank divider in your aquarium to separate the fighting goldfish.
- Add real plants to the tank to ensure a higher water quality.
- Buy rocks and other decorations in which your goldfish can hide to get away from an aggressive tank mate.